February 8 (Bloomberg) — Reducing gun violence by increasing access to mental health services may cost billions of taxpayer dollars and give drugmakers that help treat mental illness a revenue windfall. But will it reduce gun violence? The answer is uncertain.

In the wake of the tragic shooting at Sandy Hook Elementary School, there have been repeated calls to increase access to mental health services as a way to reduce gun violence in the U.S., even though the evidence is weak at best that those services actually reduce gun violence.

Absent from these calls to action is any sense of how much that policy would cost. Many argue that any cost is worthwhile if it prevents just one needless death due to gun violence. But we live in austere fiscal times and knowing the price tag before taking action makes sense.

A just-published Bloomberg Government Study estimates that the potential impact on the federal deficit due to increased spending on mental health services could exceed $260 billion from 2014 to 2021.

While the deficit could climb, this spending represents a major business opportunity for companies that make antidepressant and antipsychotic drugs. According to the study, if the percentage of people with a mental illness receiving care increased from the current level of 40 percent to 70 percent, then sales for the top 10 makers of drugs that treat mental illness would have seen revenue climb in 2012 from about $33 billion to more than $58 billion.

The policy question then becomes: what’s the goal of more mental health spending? Higher deficits, more money for drugmakers, or reducing gun violence? If it’s the latter, Congress and the president may want to rethink their approach, given the lack of evidence of its efficacy.